Researchers develop new way to generate superluminal pulses

May 03, 2012
In four-wave mixing, researchers send "seed" pulses of laser light into a heated cell containing atomic rubidium vapor along with a separate "pump" beam at a different frequency. The vapor amplifies the seed pulse and shifts its peak forward, making it superluminal. At the same time, photons from the inserted beams interact with the vapor to generate a second pulse called the "conjugate." Its peak, too, can travel faster or slower depending on how the laser is tuned and the conditions inside the gain medium. Credit: NIST

(Phys.org) -- Researchers at the National Institute of Standards and Technology (NIST) have developed a novel way of producing light pulses that are "superluminal"—in some sense they travel faster than the speed of light. The technique, called four-wave mixing, reshapes parts of light pulses and advances them ahead of where they would have been had they been left to travel unaltered through a vacuum. The new method could be used to improve the timing of communications signals and to investigate the propagation of quantum correlations.

According to Einstein's special theory of relativity, traveling in a vacuum is the universal speed limit. No information can travel faster than light.

But there's kind of a loophole. A short burst of light arrives as a sort of (usually) symmetric curve like a bell curve in statistics. The leading edge of that curve can't exceed the speed of light, but the main hump, the peak of the pulse, can be skewed forward or backward, arriving sooner or later than it normally would.

Recent experiments have generated "uninformed" faster-than-light pulses by amplifying the leading edge of the pulse and attenuating, or cutting off, the back end. The method introduces a great deal of noise with no great increase in the apparent speed. Four-wave mixing produces cleaner, less noisy pulses with a greater increase in speed by "re-phasing" or rearranging the light waves that make up the pulse.

In four-wave mixing, researchers send 200-nanosecond-long "seed" pulses of laser light into a heated cell containing atomic rubidium vapor along with a separate "pump" beam at a different frequency from the seed pulses. The vapor amplifies the seed pulse and shifts its peak forward so that it becomes superluminal. At the same time, photons from the inserted beams interact with the vapor to generate a second pulse, called the "conjugate" because of its mathematical relationship to the seed. Its peak, too, can travel faster or slower depending on how the laser is tuned and the conditions inside the laser.

In the experiment, the pulses' peaks arrived 50 nanoseconds faster than light traveling through a vacuum.

One immediate application that the group would like to explore for this system is quantum discord. Quantum discord mathematically defines the quantum information shared between two correlated systems—in this case, the seed and conjugate pulses. By performing measurements of quantum discord between fast beams and reference beams, the group hopes to determine how useful this fast light could be for the transmission and processing of quantum information.

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More information: hysical Review Letters, published online April 26, 2012.

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TopCat22
3.8 / 5 (5) May 03, 2012
since we have a speed of light in a vacuum at one rate; and we have another speed for light through a solid (glass chrystals etc)... then is it possible to travel faster than the speed of light in one medium vs another. Hence we can do all kinds of crazy things by sending information through a vacuum to reach another location before a trigger sends it through a glass and vice versa ... I don't hear of any experiments like this ... but just thinking about it can lead to interesting effects.
Husky
2.3 / 5 (3) May 03, 2012
its not faster than the absolute speed of light, just faster than the normal relative damping by the vapour medium it passes through, you could say that the pumping beam makes it more transparant.
axemaster
4.3 / 5 (7) May 03, 2012
its not faster than the absolute speed of light, just faster than the normal relative damping by the vapour medium it passes through, you could say that the pumping beam makes it more transparant.

Read it again.

In the experiment, the pulses' peaks arrived 50 nanoseconds faster than light traveling through a vacuum.

In any case you can't use this to transmit information, so it's relatively pointless.
kaasinees
1.7 / 5 (12) May 03, 2012
The pulses are made of light there fore that statement is an oxymoron...

In the experiment, the pulses' peaks arrived 50 nanoseconds faster than light traveling through a vacuum.


It either means that the equipment measurements are off(most likely) or it puts c higher than we measured before.

In no way does this experiment prove Einstein wrong.
antialias_physorg
4.6 / 5 (9) May 03, 2012
Note this part of the article:
The leading edge of that curve can't exceed the speed of light, but the main hump, the peak of the pulse, can be skewed forward or backward, arriving sooner or later than it normally would.

So we're seeing neither superluminal velocity nor superluminal information transmission here.

We're seeing here 'just' superluminal phase velocity. No photon in the pack is moving faster than light.
http://en.wikiped...velocity

Standing Bear
1.7 / 5 (12) May 03, 2012
Hey, fooling with the religion of Einstein could and can cost you your jobs, research grants, promotability at the very least. LOTS of nitwits who have long worked up to their peter principle vulnerability could become 'interested' in you in order to save themselves lest you be proven right as you inevitably will be. Better if your research was in China where you could not be fired as easy and your research would be protected by their army. Just ask Dr Ning Li who proposed a way to oppose gravity at a quantum level, and enable inertial dampers as a corollary discovery. Over ten years ago she was hounded from her job with our gov't at Huntsville, AL, and returned to China where they promptly put her into highly classified research there. Some day we may all see the results of her work, and rue that day forever.
Terriva
2.2 / 5 (13) May 03, 2012
In any case you can't use this to transmit information, so it's relatively pointless.
Never say never. For example, Lord Kelvin said, no object heavier than air can fly - and we all know, what happened next.
TS1
1.6 / 5 (7) May 03, 2012
This is probably going to contribute to another 23-page "discussion" but here goes anyway:

If something moves faster than the constant C, it should be interpreted as if moving somehow faster than "time".

It will still have a speed, for one thing. If that speed is 299,792,500 metres per second (a tad bit more than C) then it:
1. will not mean "instant" transfer
2. will not somehow go "backwards" in time.

It will still take it two seconds to traverse the distance of 599,585,000 metres.

Since we are on the subject, travelling at the speed of light would not have any effect on "time" use either. Light needs a whole year to traverse the distance of 1 lightyear, and that for a reason.
kaasinees
1.7 / 5 (11) May 03, 2012
Standing Bear seems to have a reading comprehension problem but that is okay many on this website do.
kaasinees
2.2 / 5 (13) May 03, 2012
In any case you can't use this to transmit information, so it's relatively pointless.
Never say never. For example, Lord Kelvin said, no object heavier than air can fly - and we all know, what happened next.

Now why would Kelvin say such a stupid thing? Birds fly.
axemaster
3.7 / 5 (3) May 03, 2012
It either means that the equipment measurements are off(most likely) or it puts c higher than we measured before.

Actually both of those explanations are wrong. According to an optics professor I spoke to a few days ago, this effect happens because the medium amplifies the leading edge of the slope into a peak, with the actual peak either showing up later or being suppressed. Furthermore, the amplification doesn't reproduce the original peak amplitude. So basically it's a smoke and mirrors technique.

The conclusion I would draw from this is that the amount of apparent speedup would be proportional to the width/wavelength of the pulse (I know pulses don't have well defined wavelengths but you know what I mean). So with a radio wave you could potentially set up a situation where the medium would generate a peak before you had even generated the pulse peak. Naively this would look like reverse causality, but we know better don't we!
axemaster
4.7 / 5 (3) May 03, 2012
Recent experiments have generated "uninformed" faster-than-light pulses by amplifying the leading edge of the pulse and attenuating, or cutting off, the back end.

And now that I actually read the article, all is confirmed...

Four-wave mixing produces cleaner, less noisy pulses with a greater increase in speed by "re-phasing" or rearranging the light waves that make up the pulse.

So I guess they came up with a better way. Same basic idea, anyhow.

I'd imagine that the noise is coming from the fact that they are probably doing some kind of highly amplified differentiation of the signal, with some kind of cutoff possibly using a TAC (time amplitude converter).
vacuum-mechanics
1.8 / 5 (10) May 03, 2012
By the way, it is interesting to note that constant speed of light wave c (referenced to any frame) is the assumption of Einstein in his STR. Could we make clear why he thought it is? May be the improved STR (in this paper) could guide the way.

http://www.vacuum...mid=6=en
mandel_pd
5 / 5 (2) May 03, 2012
Einstein's assumption was somewhat that the laws of physics should be the same, wether you are moving or not.
Standing Bear
1.9 / 5 (9) May 03, 2012
Kassinees needs to read the part of our rules about ethics... Duh...READ it! This is a marketplace of ideas. I happen not to believe in Einstein as it goes against common sense. But never mind that sports fans. We need also some 'true believers' as horrible examples of what used to be called 'scholasticism' in order to see how bad that bad can get. Have an original treatise that Einstein published...first edition to you book collectors..that I found in a garage sale. Believe that the old boy divided by zero somewhere...still working on that one. His math ia actually not that hard to follow. An I do not think old Al would dislike doubters. He always challenged folks to disprove him, and liked to have them around. He despised sycophants.
mandel_pd
1 / 5 (1) May 03, 2012
So Standing Bear: can you explain what's wrong with Einstein dividing by zero? (If he did it carefully)
fisycs
5 / 5 (1) May 03, 2012

Actually both of those explanations are wrong. According to an optics professor I spoke to a few days ago, this effect happens because the medium amplifies the leading edge of the slope into a peak, with the actual peak either showing up later or being suppressed. Furthermore, the amplification doesn't reproduce the original peak amplitude. So basically it's a smoke and mirrors technique.


Ran out of characters so I had to end your quote short (sorry!). Your optics professor is correct that some experiments do precisely what he described. However, this, and a number of previous experiments with "fast light" rely on "rephasing" to produce the effect. From what I understand it is interference from different wavelengths (moving with different phase velocities) interfering such that the "bulk" part of the pulse shifts forward. Also, according to the paper that is on the arxiv, the "fast" pulses are not attenuated (rather amplified, I think).
antialias_physorg
5 / 5 (2) May 04, 2012
I happen not to believe in Einstein as it goes against common sense.

So you put common sense over smart sense? That's a weird way to live...
Tachyon8491
1 / 5 (4) May 05, 2012
The leading edge of that curve can't exceed the speed of light, but the main hump, the peak of the pulse, can be skewed forward or backward, arriving sooner or later than it normally would.
What they don't allege is that the "main hump" can be made to exceed the temporal position of the leading edge of the original waveshape - if they alleged that, then it would model true superluminal propagation.
bluehigh
1 / 5 (4) May 05, 2012
vapor amplifies the seed


It modulates a secondary source so as to produce a replication of an input signal with enhanced amplitude. The original is not bigger, just distorted. So called 'early detection' of wave has been done before, now boring. The structure gets smeared and information acquisition remains luminal.
antialias_physorg
not rated yet May 05, 2012
What they don't allege is that the "main hump" can be made to exceed the temporal position of the leading edge of the original waveshape

It's stated very specifically in the articel that this isn't the case:
The leading edge of that curve can't exceed the speed of light
Terriva
1 / 5 (3) May 05, 2012
The principle of the experiment is very simple and based on the fact, during interference of three or more ripples the position of interference maxims can "travel" with arbitrary speed. At the moment, when the refraction index of the environment depends on the amplitude, such an interference makes more dense places in material, the location of whose is time dependent and it can change faster with time, than the speed of waves in the material (it can even travel in the opposite direction, for example). Now the forth wave can use these places for propagation of solitons, the effective speed of whose will surpasses the speed of all waves involved. It has nothing to do with relativity, such an effect could be reproduced in all types of materials, the density of which is proportional to energy density, like the foam
MandoZink
not rated yet May 06, 2012
I have no trouble believing this. I used to design my own sound effects circuits for my musical instruments. By adding multiple sound sources whose frequencies were offset from one another, it was possible to generate a signal pulse which essentially traveled faster down the wave on my oscilloscope. I can easily visualize a light signal traveling AT the speed of light, with an artificial pulse riding atop the signal moving forwards. Depending on the frequencies mixed, it could also be induced to move backwards.

The speed of light itself is still preserved.
MandoZink
not rated yet May 06, 2012
An addendum to my last comment: I guess it would involve interference, not mixing as in sound. Similar result though.
DarkHorse66
1 / 5 (2) May 07, 2012
"..but the main hump, the peak of the pulse, can be skewed forward or backward, arriving sooner or later than it normally would."

I haven't yet studied advanced photonics in class ('modern physics' doesn't go that far), I'm just trying to understand the distinction here, so here goes.
I get it, that the overall velocity of the pulse does not exceed the constant 'c'. However, unless I have misunderstood something, that hump being moved forward would imply at least to me that a COMPONENT of that pulse is either moving slower or faster than the rest of the sections, at least while it is being shunted from one position to another. Since the photon is already moving at 'c', does this imply temporary superluminarity for that individual component, or are we just talking about shifting the placement of the maximum amplitude of a wave? (In which case I would be happy to agree that not even individual parts of a pulse exceed 'c'.)Thanks. DH66
antialias_physorg
5 / 5 (1) May 07, 2012
that a COMPONENT of that pulse is either moving slower or faster than the rest of the sections

It only appears that way. No single photon in the pulse is going at superluminal pseeds.

Picture two waves (at the speed of light) interfering with each other so that the leading edge of the interference cancels out at some time (t equals zero). But since they are at different frequencies that interference will not hold for all times so at some future time the two will have constructive interference at the leading edge (t equals 1).

Between t equal zero and t equals 1 you will see the hump moving from the back to the front at 'apparently' superluminal speeds. No superluminal activity is involved. It only appears that way because the fields seem to cancel out (but not their derivatives! So the field component at the leading edge is actually not STATIC zero but only to first aproximation (i.e. without first derivative))

Follow the link I posted earlier for a graphic representation.
antialias_physorg
5 / 5 (1) May 07, 2012
To make it more clear: There is a photon already at the leading edge of the first wave at all times. We just don't see it at first because it destructively interferes with the leading photon from the second wave. These two photons will eventualy interfere constructively an then we will see them as the leading edge of the hump. So no photon is moved forward (at greater than c) from behind to make the hump appear.
antialias_physorg
5 / 5 (1) May 07, 2012
The real trick is to see where this doesn't constitute faster than light information transmission, because one could say:
"I'll encode my information in the phase velocity (the hump) and will therefore have information that travels faster than light!"

But remember: To create that phase velocity you have to send off two beams of light as carriers. And the hump can never travel BEYOND the leading edge of these waves of light (because there is no field there, yet, which can create constructive/destructive interference).
So your information transmission is limited to the speed at which the leading edge of the two waves travel (which is c, and not greater than c).
DarkHorse66
1 / 5 (2) May 07, 2012
@antialias: that explanation has cleared up quite a lot for me. thanks. DH66
jabailo
1 / 5 (3) May 07, 2012
Wait...this cable looks a little frayed...
El_Nose
2.3 / 5 (3) May 07, 2012
@axe

reread teh article - this can be used to transmit information the rules of relativity are not broken, the appearent peak of the wave travels faster than a normal peak would but the leading edge of the wave is equal with light in a vacuum

@ant exactly
MandoZink
5 / 5 (1) May 07, 2012
DarkHorse66 - I know that in adding sound waves, that generated pulse would not have to be part of the original signal, but an artifact of the addition of the waves - a dynamic artificial construct which would advance slowly forward.

It is conceivable to me that carefully formed light waves could induce the same effect through interference. I admit I do not know that for sure, but I can see it possible. The COMPONENT is artificially induced by the mixing of the individual waves which are traveling at C.
are we just talking about shifting the placement of the maximum amplitude of a wave

Yes, that is what I believe I am describing. Once again, this is just what I think they are implying, without fully understanding myself.

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